1. Field of the Invention
This invention relates to television standards converters.
2. Description of the Prior Art
International television programme exchange necessitates standards converters due to the different television standards used in different countries, for example, the 625-line 50-field per second PAL system used in the UK, and the 525-line 60-field per second NTSC system used in the USA.
Many different standards converters have been previously proposed, but currently one of the best known is the ACE (Advanced Conversion Equipment) developed by the British Broadcasting Corporation. Basically ACE operates on an input digital television signal line-by-line to derive interpolated samples required to form an output digital television signal. Interpolation is done not only spatially using four successive horizontal scan lines of the input television signal, but also temporally using four successive fields of the input television signal. Thus, each line of the output television signal is derived by multiplying respective samples from sixteen lines of the input television signal by respective weighting coefficients.
Further details of ACE will be found in UK patent specification No. 2 059 712 A and in "Four-field digital standards converter for the eighties" by R N Robinson and G J Cooper at Pages 11 to 13 of "Television" (the journal of the Royal Television Society) for January/February 1982.
Although ACE gives good results, there is the problem that the equipment is very bulky. This is because the interpolation function is really only one half of the total operation required for standards conversion, the other half of the operation being the re-organisation of the samples in time to form the output television signal with the correct timing. It should be understood that this re-timing of the data is quite distinct from the temporal aspect of the interpolation. In ACE both these functions are combined in a single store of four fields capacity and hence there is a necessity to access sixteen samples simultaneously for the interpolation process. To avoid clashes of reading and writing in the same element of the store, the store is configured using 4k.times.1 dynamic random access memories (RAMs). For a sampling rate of 13.5 MHz, this means that 3500 such RAMs are required. Having regard also to the necessary control circuitry, this means that ACE is of considerable size and complexity, which not only makes it costly but also creates problems of maintenance and reliability.
One object of the present invention is to provide an improved television standards converter.
Another object of the present invention is to provide a television standards converter in which the input field stores are separated from the output field stores.
According to the present invention there is provided a television standards converter comprising:
an input store for receiving an input digital television signal of one standard and deriving therefrom arrays of sixteen lines, each array consisting of four successive lines from each of four successive fields of said input television signal;
a weighting coefficient store for storing sets of sixteen weighting coefficients, respective said sets corresponding to positions both spatial and temporal of respective lines of a output digital television signal relative to said sixteen lines of said input television signal;
two interpolation filters each for deriving line-by-line said output television signal by multiplying corresponding sample values from each of said sixteen lines of said input television signal by a respective weighting coefficient in a said set of weighting coefficients and summing the resulting products to form an interpolated sample value;
an output store for receiving and storing said derived lines of said output television signal and from which said output television signal is read; and
a multi-line store connected between the output of one of said interpolation filters and said output store and used to store the additional lines derived when said output television signal has more lines per field than said input television signal.
The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings.